1. Field of the Invention
The present invention relates to a signal processing system for calibrating multiple channel signals from an array of sensors, such as microphones and antennas, having different operating characteristics.
2. Description of the Related Art
It is well known to use an array of sensors, such as microphones and radio antennas equally spaced at predetermined intervals and oriented in a specified direction for cancelling interference signals by forming a beam in the direction of arrival of a target signal. When microphones are used as the array sensors, the Griffiths-Jim beamformer is the basic technique also known as a generalized sidelobe canceller. In the Griffiths-Jim beamformer as described in a literature “Microphone Arrays,” Springer, 2001, pages 87 to 109, signals from the microphone array are combined in a fixed beamformer to enhance a desired signal and attenuate interference signals. The array beam is formed by the fixed beamfonner by linearly summing the microphone signals. If all microphones have the same operating characteristic, the summation results in an output that is M times the magnitude of each microphone signal (where M is the number of microphones). Hence the signals that arrive perpendicularly to the array surface (i.e., broadside signals) can be constructively combined. Since signals arriving in other directions have timing (phase) differences from the broadside signals, they interact with each other in a destructive manner. As a result, if the signals arriving perpendicularly to the array surface are the target signal, the target signal is enhanced, and the microphone array produces a directivity in a direction normal to its surface.
The microphone signals are also applied to a blocking matrix where these signals are combined to obtain multiple interference references. The enhanced target signal is delayed for an interval corresponding to the time taken by the blocking matrix to perform the matrix calculation. The delayed beamformer output and the interference references are combined in a multi-channel canceller. In the multi-channel canceller the interference references are used as interference replicas for subtraction from the enhanced target signal to produce an enhanced target signal.
However, if the operating characteristics of the microphones are not equal to each other due to their variation, the microphone signals partially interact with each other in a destructive way. This results in the array producing a degraded directivity in the direction normal to its surface. A similar problem occurs in the blocking matrix. In this case, it is the target signal that finds a leakage path to the outputs of the blocking matrix. This results in the multi-channel canceller performing partial cancellation of the target signal and causes a distortion in its output signal.
The element imperfection problem, caused by the above-mentioned variation, is addressed by a calibration technique described in IEEE Transactions on Signal Processing, Vol. 42, No. 10, pages 2871-2875, October 1994. According to this technique, a blocking matrix is designed based on optimal eigenvector constraints by using a broadband signal and a fixed beamformer corresponding to the blocking matrix is then designed. However, the use of broadband signal requires that individual calibration is necessary for each microphone array in advance of its manufacture. This is disadvantageous for quantity production.
Another calibration technique, as described in IEEE Transactions on Antennas and Propagations, Vol. 34, No. 8, pages 996-1012, August 1986, introduces noise at an appropriate level to each microphone signal. However, this prior art requires precision setting of the added noise level. Data such as signal-to-noise ratios, interference-to-signal ratios and the level of noise at each microphone must be additionally calculated on a real-time basis. The amount of computations is substantial and the additive noise is a potential source of poor sound quality. Further calibration techniques are disclosed in a number of patent publications. Japanese Patent Publication 2004-343700 discloses a technique that uses a calibration speaker and a signal processing system and Japanese Patent 3337671 teaches the use of a calibration microphone. Japanese Patent Publication 2002-502193 employs multiple adaptive filters for respective microphones. However, these prior art techniques require separate devices that present an increase in hardware. According to a further technique disclosed in Japanese Patent Publication 2004-502367, the output of a single microphone is used as a reference level. However, this reference level must be supplied from an external source.